292 THE ORIOiy OF SPECIES 



possessors, and therefore cannot have been influenced 

 through natural selection. Bronn adduces the length of 

 the ears and tails in the different species of hares and 

 mice — the complex folds of enamel in the teeth of many 

 animals, and a multitude of analogous cases. With respect 

 to plants, this subject has been discussed by Nageli in 

 an admirable essay. He admits that natural selection has 

 effected much, but he insists that the families of plants 

 differ chiefly from each other in morphological characters, 

 which appear to be quite unimportant for the welfare of 

 the species. He consequently believes in an innate ten- 

 dency toward progressive and more perfect development. 

 He specifies the arrangement of the cells in the tissues, 

 and of the leaves on the axis, as cases in which natural 

 selection could not have acted. To these may be added 

 the numerical divisions in the parts of the flower, the 

 position of the ovules, the shape of the seed, when not 

 of any use for dissemination, etc. 



There is much force in the above objection. Never- 

 theless, we ought, in the first place, to be extremely 

 cautious in pretending to decide what structures now 

 are, or have formerly been, of use to each species. In 

 the second place, it should always be borne in mind 

 that when one part is modified, so will be other parts, 

 through certain dimly seen causes, such as an increased 

 or diminished flow of nutriment to a part, mutual press- 

 ure, an early developed part affecting one subsequently 

 developed, and so forth — as well as through other causes 

 which lead to the many mysterious cases of correlation, 

 which we do not in the least understand. These agencies 

 may be all grouped together, for the sake of brevity, 

 under the expression of the laws of growth. In the 



